The present invention relates to a treatment system for percutaneous coronary angioplasty or peripheral angioplasty in which a dilation catheter is used to cross a lesion in order to dilate the lesion and restore normal blood flow in the artery. It is particularly useful when the lesion is a calcified lesion in the wall of the artery. Calcified lesions require high pressures (sometimes as high as 10-15 or even 30 atmospheres) to break the calcified plaque and push it back into the vessel wall. With such pressures comes trauma to the vessel wall which can contribute to vessel rebound, dissection, thrombus formation, and a high level of restenosis. Non-concentric calcified lesions can result in undue stress to the free wall of the vessel when exposed to high pressures. An angioplasty balloon when inflated to high pressures can have a specific maximum diameter to which it will expand but the opening in the vessel under a concentric lesion will typically be much smaller. As the pressure is increased to open the passage way for blood the balloon will be confined to the size of the opening in the calcified lesion (before it is broken open). As the pressure builds a tremendous amount of energy is stored in the balloon until the calcified lesion breaks or cracks. That energy is then released and results in the rapid expansion of the balloon to its maximum dimension and may stress and injure the vessel walls.
Recently, a new system and method has been contemplated for breaking up calcium deposits in, for example, arteries and veins. Such a system is described, for example in U.S. Patent Publication No. 2009/0312768, Published Dec. 17, 2009. Embodiments described therein include a catheter having balloon, such as an angioplasty balloon, at the distal end thereof arranged to be inflated with a fluid. Disposed within the balloon is a shock wave generator that may take the form of, for example, a pair of electrodes, which are coupled to a high voltage source at the proximal end of the catheter through a connector. When the balloon is placed adjacent a calcified region of a vein or artery and a high voltage pulse is applied across the electrodes, a shock wave is formed that propagates through the fluid and impinges upon the wall of the balloon and the calcified region. Repeated pulses break up the calcium without damaging surrounding soft tissue.
Each high voltage pulse causes an arc to form across the electrodes. The arc in turn causes a steam bubble to form. Each steam bubble has the potential of producing two shock waves, a leading edge shock wave as a result of bubble expansion and a trailing edge shock wave as a result of bubble collapse. The trailing edge shock waves exhibit highly variable energy levels and generally, much greater energy levels than the leading edge shock waves. The energy levels of the trailing edge shock waves are substantially dependent on the uniformity of the bubble collapse. The uniform collapse of spherical bubbles to a point appears to create the highest shock wave energies. Unfortunately, spherical bubble configuration requires a substantially larger space than is available in a balloon that must fit into a calcified vein or artery or even a ureter. In fact, the trailing edge shock wave can be substantially eliminated by confining the bubble to an irregular shape. As a result, for angioplasty or other cardiac and non-cardiac applications of shock waves, the trailing edge shock wave cannot be reliably relied upon to produce consistent results.
However, the leading edge shock waves formed by bubble expansion are a different matter. While exhibiting generally lower energies, they are more consistent in energy level. As a result, leading edge shock waves are good candidates for use in medical procedures such, for example, angioplasty or valvuloplasty.
Another consideration is the amount of energy represented by the high voltage applied to the electrodes. Each high voltage pulse removes a portion of the electrode material. Since the size of the electrodes must be small in order to fit into the calcified vein or artery, they are only capable of sustaining a limited numbers of high voltage pulses sufficient to form the shock wave resulting electrical arc.
Also, it has been learned that to sustain a leading edge shock wave, it is not necessary to sustain the high voltage throughout the shock wave. Sustaining the high voltage beyond some point after the initial arc does not lead to shock waves of any greater intensity. Further, since the bubbles are formed of steam, the steam produces heat which can increase the temperature of adjacent soft tissue. Just a two degree Celsius elevation in temperature above body temperature can result in tissue damage.
A still further important aspect of prior art attempts to use shock waves from electrical arcs for therapeutic purposes is that from the time the high voltage is first applied to the electrodes to the time in which the arc occurs there is a dwell time (Td) that is highly variable from one high voltage application to the next. To account for the dwell times that are long, prior art strategies have relied upon high voltage applications where all high voltage pulse durations or pulse widths are of the same length and of a length sufficient to extend through the longest of the anticipated dwell times plus the associated arc and steam bubble. As a result, when the dwell times are shorter than the maximum, the high voltage application durations are longer than necessary and can unnecessarily extend the arc and the steam bubble well beyond a time required to produce a shock wave of maximum intensity. The result is wasted energy, extended electrode erosion, and unnecessary heating of the adjoining tissue.
Hence, there is a need in the art to be able to control the energy applied to the electrodes of an electrical arc shock wave generator. More particularly, there is a need to control the applied energy to assure appropriate bubble and shock wave formation while at the same time conserving electrode material and assuring tissue safety. The present invention addresses these and other issues.